1. Field of the Invention
The present invention relates to the field of bandgap temperature sensors.
2. Prior Art
Bandgap temperature sensors are well known in the prior art. Such temperature sensors are based on the variations of a pn junction forward conduction voltage drop with temperature of and current density in the pn junction. In one form, two, typically identical transistors are operated with different currents there through, and accordingly with a correspondingly different current density. An output is provided proportional to the difference in the forward conduction voltage drop of the two transistors, which output is proportional to absolute temperature. In another form, a form particularly of interest in the present invention, utilizes a single pn junction such as a diode or diode-connected transistor, operating first at one current and thus one current density, and then at a second current and thus a second current density different from the first current density. The difference in the forward conduction voltage drop under the two operating conditions is also proportional to absolute temperature.
In either event, the base-emitter voltage, or the forward conduction voltage drop across a pn junction diode, may be expressed by the equation: EQU V=V.sub.go (1-T/T.sub.0)+V.sub.BE0 (T/T.sub.0)+nKT/q ln(T.sub.0 /T)+KT/q ln(I.sub.C /I.sub.C0)
where:
V.sub.BE =voltage drop across the pn junction PA2 V.sub.go =semiconductor bandgap extrapolated to a temperature of absolute zero PA2 V.sub.BE0 =pn junction V at temperature T.sub.0 and corresponding current I.sub.C0 PA2 q=electron charge PA2 n=structure factor
When two identical pn junctions are operated with current densities J1 and J2, or alternatively as in the present invention, a single pn junction is first operated with a pn junction current of J1 and then operated with a pn junction current of J2, the difference in the pn junction forward conduction voltage drops .DELTA.V from the above equation may be expressed as: EQU .DELTA.V=(KT/q) ln(J.sub.1 /J.sub.2)
Thus, the difference in the forward conduction voltage drops is directly proportional to absolute temperature. It may also be seen from the foregoing equation, however, that the scale factor for the sensitivity of the difference in the forward conduction voltage drops with temperature is dependent on the ratio of the current densities. If the ratio is not well controlled, then the scale factor of the temperature sensitivity of the difference in the forward conduction voltage drops in microvolts per degree centigrade is also not well controlled. In the prior art, the two different but proportional currents were generated with a current mirror using transistors of substantially different size, so that one current would only be a fraction of the other current. Since the ratio of the two currents is primarily transistor size dependent, reasonable control of the current ratios could be obtained. If greater accuracy was required, circuit trimming could be done at the wafer stage to provide a more accurate current ratio. Even then, however, the two currents would not track each other that well over the required temperature range, so that even with the expense of circuit trimming, the accuracy of the temperature measurement is limited. One aspect of the present invention is the provision of currents through a pn junction temperature sensor which effectively have a precise current ratio independent of processing variations and temperature variations in the current sources.
Also in the prior art, when two successive different currents are provided through a single pn junction for temperature measurement purposes, the measurement will be in error if the temperature of the pn junction changes between the time the first current is provided there through and the time the second current is provided there through. In the present invention, this problem is overcome, allowing a temperature measurement to accurately reflect, by way of example, the temperature at the beginning of a measurement cycle, at the end of the measurement cycle, or midway through the temperature measurement.